Winter greenhouse vegetable growing in the upper Midwest

by Bill and Mary Weaver
There are a surprising number of what are called “deep winter greenhouses,” in Minnesota — well over a dozen — with more in the planning stages. They are used for growing mostly cool season greens and some root vegetables for sale to CSA’s, restaurants and Co-ops, straight through Minnesota’s brutal winters. Quite a few of them, because they focus on using alternative energy sources for heat, have received grants from a Minnesota groups called CERTS, Clean Energy Resources Teams. The University of MN Extension is providing horticultural assistance, and several deep winter greenhouse grower organizations have formed around the state, holding workshops where growers and prospective growers can learn and compare notes.
The most daring of these straight-through-the-winter greenhouses we’ve encountered has been Whitewater Gardens Greenhouse in Altura, MN where there are excellent markets for the fresh, local wintertime vegetables they grow in two area winter farmers markets, two co-ops, and several restaurants in two sizeable cities.
Sandy and Lonny Dietz chose to build a freestanding (rather than earth-bermed on the north), geothermally heated greenhouse to grow high value crops like greenhouse tomatoes, European cucumbers, bell peppers and green beans, warm season crops that require a greenhouse temperature of about 70 degrees, which is a real challenge in Minnesota’s deep winter, -30°F temperatures with high winds.
After extensive research and feasibility studies, the Dietz’s built their 46×126’, two bay, inflated double poly greenhouse with the geothermal heating tubes placed about 14 inches below the surface of the soil, effectively heating the root zone. Like most deep winter greenhouses in Minnesota, theirs was constructed with heavy insulation four feet deep in the soil around the perimeter of the greenhouse and with extra care to make the construction tight.
The Dietz’s chose a geothermal system with three heat pumps because it could also be used for cooling their produce, both fresh picked produce in the hot summers, and for long-term storage vegetables during the fall, winter and spring.
“With the geothermal system, we can keep our main cold storage at 38 degrees. At the same time, during the summer, the heat drawn from the coolers heats the soil in the greenhouse, so that in the fall, we’re starting with soil around 65 degrees, rather than the usual 45 degrees, giving us a jump start.”
Despite the feasibility studies, they have had a learning curve along the way. “We were aware that in trying to grow in the ground, we would be attempting something quite unusual here. Most geothermal greenhouses have the tubing in concrete. Our advisors had no experience with this type of system. We have since learned that with a geothermal system in Minnesota, you can heat the soil, or the ambient air, but not both. In retrospect, a forced air system would have been more efficient,” Lonny Dietz continued.
The Dietz’s had two exceptionally rough winters. Two years ago, when propane had hit $2 a gallon and climbing, peaking at $4, Dietz simply closed down the greenhouse. This year, however, he is well prepared, having purchased a 2.5 million BTU biomass burning system. “This should heat the ambient air economically in this greenhouse as well as several smaller greenhouses, including one built for starting plants and another for growing cool season greens.” The latter will be held at about 50°F.
Until this winter, their main greenhouse, as mentioned, was double walled, 6 mil, inflated poly. “It was tricky to keep it inflated. We had so many separate inflation zones, each with its own fan. Each side wall was a separate inflation zone, and we had four zones in the roof in each of the bays. The roof of each bay was built with four separate inflation zones so it could be vented naturally through the roof peaks, avoiding the use of the usual 4-foot venting fans on either end.”
To avoid running all those inflation fans, this year for the first time, the Dietz’s wrapped their entire greenhouse in Sola Wrap, a product similar to the bubble wrap that is used for shipping fragile items, which has the insulating air bubbles built into the wrap.
Tomato varieties constituted another learning curve to be navigated. “Most greenhouse tomatoes are bred for hydroponic systems,” continued Dietz, “not an in-ground system like ours. We tried more than 20 different varieties to find one that would perform well in our greenhouse and also have good flavor.
“The first year we tried several rows of heirloom tomatoes,” which are a very popular item in their area. “We have not found an heirloom variety yet that likes our system,” continued Dietz. “Heirlooms haven’t set enough fruit to make them worthwhile. So now we grow all hybrids. But many of the hybrid varieties seemed to be hit or miss. They would do very well one season, but yield poorly the next.”
This year, with the large capacity biomass heating system to heat the ambient air and supplement the geothermal system buried in their soil, the Dietz’s appear poised to harvest a good crop. There are many area markets eager for their fresh, local, vine-ripened tomatoes, cukes, green beans and bell peppers in the middle of a Minnesota winter. “We have eight double rows in the large greenhouse, and at any given time, five of those will be in tomatoes.
“We’re hoping to sell everything within a 50-mile radius of our greenhouse,” Dietz continued, “but if production is really good, we have friends who could take orders for our products in Minneapolis and deliver them along with their own winter storage crops.”
The Dietz’s also have ‘Bolero’ carrots, which are excellent keepers, and several kinds of potatoes in long- term cold storage to sell with their greenhouse-grown items, along with stored sweet potatoes (all are farm specialties) in a warmer storage, and cabbage in short-term cold storage, all cooled by their geothermal system. They appear to have traversed the sometimes-difficult learning curve.
On their wish list is a rainwater system. “We still have some limitations on our available water system, which has a naturally high pH,” Lonny Dietz concluded.